The invention relates to a method for detecting the point of immersion or emergence of a solid or porous body into or out of a liquid. The last mentioned process criterion plays an important part in chemical process engineering when, in liquid/solid separation processes such as, for example, decanting, filtration or vacuum filtration, the point in time of the liquid level entering the solid cake is to be detected as exactly as possible. For example, the treatment of the filter cake with a washing liquid or conditioning liquid can then be carried out in a controlled manner. "Controlled" here means that the quantity of washing liquid is minimized and, on the other hand, excessive drying of the solid is avoided.
Moreover, during filling or emptying of vessels, exceeding of or falling below a certain liquid level is to be detected.
Ultrasonic sensors, laser sensors and radiometric sensors are known for the scanning of liquid levels. However, these are designed above all as liquid level indicators and do not give an adequate measured signal when a surface is flooded or a liquid passes through a surface of a solid.